Good point, dirac. In addition, Austen's heroines (and their prospective mates) seem to possess integrity,sincerity, and the ability to see through false pretensions. This inevitably makes them somewhat uncomfortable with and skeptical about conventional social charm, ironic about it, and -- sooner or later -- subversive of it. What conventional society call "charming" all too often turns out to be false, deceptive, superficial, smug, and/or manipulative.
Hmmm....it seems to me that Austenís heroes tend to be lacking in the charm department: charm of manner is not a striking quality in Darcy, Knightley, Edmund Bertram, or Wentworth, although they are attractive in other ways. Notable charm in Austen tends to be a sign of villainy or carelessness in men (though not necessarily in women; Elizabeth Bennet is arguably the most charming heroine in literature).
Oscar Wilde was speaking for conventional society when he wrote (more or less): One of the great charms of marriage is that it makes a life of deception necessary for both parties." Austen's value system is quite different. Maybe that's one of the qualities that makes her seem "modern" today.